1. Field of the Invention
The present invention relates to a hyper-fine cement, in particular a hyper-fine cement having a particle size D50 below 1 μm, and also a process for producing this hyper-fine cement and its use.
2. Discussion of Background Information
Cements such as portland cements are usually produced from natural raw materials by means of a firing process. This forms clinkers. Milling of the clinkers in a dry milling process gives a fine powder, namely the cement. Clinkers have to be milled dry since otherwise the cement formed will set and become unusable. Cements are hydraulic. This means that when mixed with water they set to give a compact moulded article; in this process, the important building block of clinkers, namely monocalcium silicate, forms tricalcium silicate which crystallizes in fine needles which intermesh and thus bring about the strength of the cement block.
Cement mortars are aqueous mixtures of milled cement with sand. Concretes are aqueous mixtures of milled cement with relatively coarse gravel.
The particle size of the milled clinkers is usually above 10 μm. Since the particle size is relatively non-uniform, it is usual to indicate the percentages which are below a particular size. A measure of the fineness which has become established in the cement industry is the Blaine value. Here, a particular amount of air is pumped through a standardized bed of milled cement and the time required for this is measured. The finer the particles, the longer the time required. Another value is the D50. It indicates the percent by weight of the particles smaller than a given diameter. The finer the clinkers are milled, the greater the strength of the mortar or concrete generally becomes.
In the milling of cement, it is essential that the cement is milled dry, i.e. in the absence of water. Dry milling processes have a distinct lower limit to the particle size which can be achieved, since in the dry state the recombination rate of the particles is significantly higher than, for example, in wet milling processes since charges are formed on the surfaces which have been broken apart and quickly rejoin the particles.
Commercial portland cement has an average particle size distribution having a D50 in the order of 70 μm. Cements having a D50 of 10 μm or somewhat below are referred to as ultrafine cements; commercial ultrafine cements have a D50 in the range from 3 to 8 μm, see FIG. 1. They are obtained from normal cements by separating off the coarser fractions by means of various separation processes.
Ultrafine cements are used, in particular, as additives to coarser mixtures in the petroleum and natural gas industry. Thus, for example, mortar based on ultrafine cement has gained increasing importance in recent years in rock and soil injections for strengthening or sealing. For use of ultrafine cements in the oil and gas industry, these grades should have an excellent penetration capability and excellent strength and keeping qualities so that they can be used for the pressure cementing of oil well matrices or formations, in particular for the control of gas and water (GOR (gas-oil ratio) and WOR (water-oil ratio)).
To produce ultrafine cements, it is usual to employ dry milling since this can be carried out simply and quickly but it requires careful matching of the milling conditions to the respective sample. Apart from dry milling processes, wet milling processes using water are also known, as described, for example, by Naudts, A., Landry E. “New On-site Wet Milling for the preparation of Ultrafine Cement-based Grouts”, 3rd International Specialty Conference on Grouting and Ground Treatment, 10-12 Feb. 2003, New Orleans, La., USA. However, such wet milling using water is only possible if the ultrafine cement is produced on site in situ immediately before injection or treatment, before it becomes hard.
Other processes for producing ultrafine cement are a process in which ultrafine additives having finer particles are added to the cement (Clarke, B., “Performance characteristics of microfine cement”, ASCE preprint 84-023, Atlanta, Ga., American Society of Civil Engineers, New York) and a process in which fine by-products from normal cement production stages are collected (U.S. Pat. No. 5,417,760). As additives which improve some mixing properties and reduce the Blaine value of the cement, slag, fine silica and fly ash are mentioned by Naudts, A., Landry E., Hooey, S., Naudts, W., “Additives and Admixtures in Cement-based Grouts”, 3rd International Specialty Conference on Grouting and Ground Treatment, 10-12 Feb. 2003, New Orleans, La., USA.
Ultrafine cements serve to strengthen or seal loose or porous rock; penetration of the cement made up with water into relatively small pores should also be achieved. However, since many rocks have pores in the lower micron range or even below 1 μm, the process cannot be used in such cases and is therefore very limited overall. In the exploitation of natural gas reservoirs, in particular, this is serious because gas can very easily escape from the rock through even very fine pores and very high losses therefore occur in many natural gas wells.
The object was to provide a process for sealing or strengthening porous moulded articles, rock or porous formations, by means of which even very fine pores of rock can be sealed so that even natural gas under pressure cannot escape. In particular, a cement which can be pumped into even very fine pores of rock and hardens there should be provided.
The object has surprisingly been able to be achieved by a process for producing a hyper-fine cement having a particle size D50 of <1 μm, which comprises the wet milling of cement or cement clinker in a nonaqueous solvent. Since the cement having a D50 below 1 micron which is obtained by the process of the invention can penetrate into even very fine pores, it is of extraordinarily high importance for the sealing of rock, in particular in the case of natural gas wells. The invention is described in detail below.